1. Field of the Invention
The present invention relates to a printer used for forming images on a photosensitive recording medium by a photosensitive system.
2. Description of the Related Art
An image captured by a digital camera, for example, can be formed on an ordinary paper based on the digital data by an ink jet system or a thermal transfer system. It is also considered to record such an image on a photosensitive film based on the digital data by a photosensitive system. In the photosensitive system, an image is formed by exposing a photosensitive film to light followed by developing the film. Therefore, an image forming apparatus utilizing this system can be made compact relatively easily as compared with one utilizing the ink jet system or the thermal transfer system. For this reason, a digital camera has been commercially introduced which incorporates a print head of a photosensitive type for printing an image immediately after capturing the image. For easier carriage of the digital camera, it is necessary to reduce the size of the print head as well as other parts of the camera.
In forming an image on a photosensitive film by the photosensitive system, for example, the photosensitive film is irradiated with light in the form of a line extending in the primary scanning direction and the irradiation region is shifted in the secondary scanning direction for scanning the entirety of the photosensitive film. As the print head for emitting light in the form of a line, use may be made of one including a plurality of light emitting elements (point light sources) aligned in a row extending in the primary scanning direction. As the light emitting elements, light emitting diodes are typically used. However, organic EL light emitting elements may alternatively be used. An organic EL element means an element which emits light by electroluminescence when electric field is applied to a light emitting layer containing an organic material.
However, light emitting elements deteriorate with a lapse of time, reducing the amount of light emitted. Particularly, EL light emitting elements are likely to deteriorate due to the formation of impurities or entering of water in the light emitting layer. Further, the plurality of light emitting elements do not deteriorate to a same degree with a lapse of time and differ from each other in speed of deterioration. Therefore, when one light emitting element deteriorates to a considerably large degree (thereby emitting little amount of light) as compared with others, it is impossible to irradiate the photosensitive film properly with linear light. In such a case, when the print head is moved in the secondary scanning direction to irradiate the entire photosensitive film with light, a portion of the photosensitive film extending in the secondary scanning direction is left insufficiently irradiated with light. This portion appears as a line in the formed image. This also means that a print head has a short lifetime when a light emitting element such as an organic EL light emitting element which is likely to deteriorate is utilized.
Although, an LED is unlikely to deteriorate as compared with an organic EL light emitting element, its power consumption is disadvantageously higher than that of the organic EL element. Therefore, when a plurality of LEDs are used as a light source of a print head, its power consumption becomes high. Since the printer of a digital camera as a portable device typically uses a low-capacity dry cell or rechargeable battery as the light source, the power consumption need be decreased.
The present invention aims to provide a print head for irradiating a photosensitive recording medium with light, which is firstly capable of preventing deterioration of a formed image due to the degradation of the light source for forming a proper image, which secondly has a long lifetime, and which thirdly has a small size and low power consumption.
According to the present invention, there is provided a print head comprising an illuminator for emitting light in a line extending in a primary scanning direction, a liquid crystal shutter for selecting whether or not light traveling from the illuminator is allowed to pass and, and a light emitting portion for emitting light traveling from the liquid crystal shutter toward a photosensitive recording medium.
With such a structure, after light emitted from the illuminator becomes incident on the liquid crystal shutter, the light passing through the liquid crystal shutter is emitted from the light emitting portion. Thus, the liquid crystal shutter can define the state of light (amount, wavelength and the like) to be emitted from the light emitting portion. Therefore, even when the light source device includes a portion emitting a smaller amount of light, for example, and hence variation exists in the amount of light, the liquid crystal shutter can eliminate such variation.
For example, the liquid crystal shutter may include a plurality of individual shutter portions aligned in the primary scanning direction. In this case, each of the shutter portions is capable of individually selecting whether or not the light traveling from the illuminator is allowed to pass.
For example, the illuminator may emit light (e.g. white light) which includes red light, green light and blue light. Specifically, the illuminator may be provided with a light emitting portion in the form of a strip extending in the primary scanning direction or a plurality of point light emitting portions aligned in a row extending in the primary scanning direction. For performing color printing using such an illuminator, the plurality of shutter portions may include a plurality of first shutter portions aligned in a row extending in the primary scanning direction for selectively passing red light, a plurality of second shutter portions aligned in a row extending in the primary scanning direction for selectively passing green light, and a plurality of third shutter portions aligned in a row extending in the primary scanning direction for selectively passing blue light.
The liquid crystal shutter may include a plurality of first electrodes arranged adjacent to each other, a plurality of second electrodes arranged adjacent to each other and extending transversely to the first electrodes, and a liquid crystal layer provided between the first electrodes and the second electrodes. In this case, the transverse portions of the first and the second electrodes correspond to the first through the third shutter portions.
With such a structure, for irradiating the photosensitive recording medium with red light for example, a shutter portion through which red right is to pass is selected from the first shutter portions depending on the image to be formed, and light is allowed to pass through the selected first shutter portion. For the selected first shutter portion, a voltage is applied to the liquid crystal between the first electrode and the second electrode constituting the first shutter portion. At that time, when a non-selected first shutter portion through which red light should not pass exists adjacent to the selected first shutter portion, a potential difference is generated between the adjacent first electrodes or between the adjacent second electrodes constituting these shutter portions. Such a potential difference is more likely to be generated as the distance between the electrodes (between adjacent shutter portions) decreases. When the potential difference is generated between the adjacent electrodes, the alignment of liquid crystal nearby is disturbed. As a result, the light component of green light or blue light, for example, may unintentionally pass through the liquid crystal shutter.
For dissolving such a problem, it is preferable that the first shutter portions, the second shutter portions and the third shutter portions are respectively arranged in a plurality of rows, and that the shutter portions in each row are disposed in staggered relationship with the shutter portions in an adjacent row. With such an arrangement, a relatively large distance can be kept between adjacent shutter portions. Therefore, the disturbance of liquid crystal around the non-selected shutter portion can be avoided, which prevents unintended light from passing through the liquid crystal shutter for emission from the print head.
For arranging the first through the third shutter portions in staggered relationship in two rows, the liquid crystal shutter may be structured as follows. That is, the plurality of first electrodes includes a pair of electrodes for red light, a pair of electrodes for green light and a pair of electrodes for blue light, and each of the second electrodes includes a plurality of main overlapping portions which overlap one of the paired electrodes for red light, one of the paired electrodes for green light or one of the paired electrodes for blue light, and a connecting portion connecting adjacent ones of the main overlapping portions. Preferably, the connecting portion is smaller in width than the main overlapping portions. In this case, the main overlapping portions correspond to the first through the third shutter portions.
Preferably, the liquid crystal shutter is adapted for driving in OCB mode. In this case, the liquid crystal shutter includes a first transparent substrate, a second transparent substrate arranged in facing relationship to the first transparent substrate, and liquid crystal retained between the first and the second transparent substrates so as to keep splay alignment when no voltage is applied. In this case, the liquid crystal shutter includes a phase compensation film laminated on at least one of the first and the second transparent substrates. When the OCB mode is utilized, the state of the liquid crystal readily changes in response to the change of the voltage application, which realizes high-speed printing.
The print head of the present invention may further comprise control means for driving the liquid crystal shutter. Preferably, the control means operates for applying a voltage to the liquid crystal which is higher than a minimum transition voltage required for causing transition of the liquid crystal from splay alignment to bend alignment. For example, the liquid crystal shutter includes at least one first electrode formed on the first transparent substrate and at least one second electrode formed on the second transparent substrate. In this case, at least one first electrode and at least one second electrode are utilized for applying voltage to the liquid crystal. In causing transition of the liquid crystal from splay alignment to bend alignment, the control means applies an AC voltage to the first electrode while applying an AC voltage to the second electrode to provide an AC waveform having a same cycle as and 180-degrees phase-shifted from that of the AC voltage of the first electrode, a voltage applied across the liquid crystal being higher than the minimum transition voltage.
In the OCB mode, after the transition of the liquid crystal from the splay alignment to the bend alignment is performed, the actual driving is performed in the bend alignment state. When a high voltage is applied during the transition, the time required for the transition is shortened, which leads to the shortening of the time required for printing.
The liquid crystal shutter may comprise TN liquid crystal retained between the first and the second transparent substrates. In such a case, it is preferable to add cyanide as a chiral dopant. In such a case, the viscosity of the liquid crystal reduces so that the state of the liquid crystal readily changes in response to the change of the voltage application, which realizes high-speed printing.
Preferably, cyanide may be added in an amount of 0.1-4.0 parts by weight relative to 100 parts by weight of liquid crystal, and the viscosity of the liquid crystal may be 10-20 mPaxc2x7s.
The liquid crystal shutter may comprise a pair of transparent substrates and ferroelectric liquid crystal or antiferroelectric liquid crystal retained therebetween. Ferroelectric liquid crystal or antiferroelectric liquid crystal is highly responsive to the change of the state of voltage application. Therefore, when such liquid crystal is used for the liquid crystal shutter, the ON/OFF operation of individual shutter portions can be performed with high responsiveness, which realizes high-speed printing.
For the illuminator, use may be made of one that can individually emit red light, green light and blue light. For example, the illuminator includes a red light source for emitting red light in a line, a green light source for emitting green light in a line, and a blue light source for emitting blue light in a line. In this case, each of the red light source, green light source and blue light source may be a linear light source in the form of a strip or may comprise a plurality of point light sources aligned in a row. For individually emitting red light, green light and blue light, these colors of light may be successively emitted. Alternatively, these colors of light may be emitted at the same time to emit white light, and red, green or blue light may be taken out by the use of a liquid crystal shutter.
The illuminator may be provided with an organic light source including a light emitting layer containing an organic material. The organic material emits light by electroluminescence when electric field is applied.
As described above, a light emitting element utilizing organic EL is more likely to deteriorate as compared with an LED light source. Therefore, the present invention, which is capable of reducing the influence of deterioration of the illuminator (light emitting element), is useful for a print head with a light source utilizing organic EL. Since a light emitting element utilizing organic EL has low power consumption, the use of such a light emitting element can decrease the power consumption of the print head.
Preferably, the organic light source may be covered with a sealing portion formed of an inorganic insulating material.
With such an arrangement, the organic light source is protected from an external force. Since an inorganic compound is generally less likely to absorb water as compared with an organic compound, the sealing portion can prevent water from the surroundings from entering the illuminator. When water is prevented from entering the illuminator, the deterioration of the light source can be suppressed even when the light source includes a light emitting layer containing an organic material. Therefore, it is possible to prolong the lifetime of the light source and hence the lifetime of the print head.
For example, the illuminator may include a light source device including one or a plurality of point light sources, and a light guide for guiding the light emitted from the point light sources for emission in a line extending in the primary scanning direction.
Since this structure utilizes a light guide, the photosensitive recording medium can be irradiated with linear light without aligning light emitting elements (point light sources) in a row. As a result, irradiation of the photosensitive film is possible even with a small number of light sources. Therefore, the power consumption of the print head can be decreased even with the use of an LED as the light source. When the LED is used as the light source, deterioration of the image quality due to the deterioration of the light source can be prevented, which leads to a prolonged lifetime of the print head.
For example, the light guide has a bar-like configuration extending in the primary scanning direction. The light guide may include a light incident surface for guiding light therein, and a light reflecting surface, and a light emitting surface spaced thicknesswise from the light reflecting surface. Preferably, the light incident surface is provided at an end portion of the light guide. The light reflecting surface includes a plurality of inclined surfaces inclined toward the light incident surface for making light traveling from the light incident surface emit from the light emitting surface.
For example, the plurality of inclined surfaces are provided by forming a plurality of recesses at an obverse surface of the light guide. The plurality of inclined surfaces may be equal or substantially equal to each other in angle of inclination, for example. Preferably, the plurality of recesses have progressively increasing depths away from the light incident surface. With this structure, a farther portion from the light incident surface receives a larger amount of light, which eliminates variation of the amount of light in the primary scanning direction.
The light guide may include a plurality of additional inclined surfaces for guiding light reflected at an end surface located opposite to said end portion toward the light emitting surface. For the light reflected by the end surface opposite to the end on the light incident side, the light is more likely to be reflected by the additional inclined surfaces at a portion farther from the light incident surface. Therefore, a large amount of light can be obtained at a portion far from the light incident surface, so that variation of the amount of light in the primary scanning direction can be eliminated.
Preferably, the light guide is covered with a light shield for absorbing light emitted from the light guide. The light shield prevents light traveling from the illuminator from being emitted toward portions other than the liquid crystal shutter. Preferably, the light shield is formed with an opening extending in the primary scanning direction for emitting light therethrough, and the light shield includes a first light shielding portion covering the light emitting surface of the light guide and a second light shielding portion covering portions of the light guide other than the light emitting surface. In this way, it is preferable to cover the light guide as much as possible by the light shield except the portion contributing to the light emission toward the liquid crystal shutter.
Preferably, the light guide is covered with a reflector for returning light exiting the light guide into the light guide. With such a structure, light emitted from the light source is efficiently utilized. The reflector may be covered with a light shield for absorbing light passing through the reflector.
The plurality of point light sources include a red point light source for emitting red light, a green point light source for emitting green light and a blue point light source for emitting blue light, for example. In this case, the light source device includes a substrate on which the red point light source, the green point light source and the blue point light source are mounted, and a plurality of wirings formed on the substrate.
Preferably, the red point light source, the green point light source and the blue point light source are aligned in a row extending in the secondary scanning direction. In this case, the substrate and the light incident surface face each other while standing upright with respect to the light emitting surface. With such a structure, the row of three kinds of point light sources extends perpendicularly to the thickness direction of the light guide. Therefore, the use of three kinds of light sources does not increase the dimension of the substrate in the perpendicular direction (width of the substrate), so that the thickness of the light source device including the light guide can be decreased.
For example, each of the red point light source, the green point light source and the blue point light source includes a first electrode and a second electrode. The plurality of wirings are formed on a surface of the substrate on which the point light sources are mounted, and the wirings include a first wiring electrically connected to the first electrode via a conductor wire and a second wiring electrically connected to the second electrode. Preferably, in this case, the conductor wire extends obliquely to a direction perpendicular to the row of the light sources. When the conductor wire is arranged to extend obliquely to a direction perpendicular to the row of the light sources, the width of the substrate and hence the thickness of the light source device can be prevented from increasing.
For example, each of the red point light source, the green point light source and the blue point light source is capable of being driven individually. That is, in the print head of the present invention, the red point light source, the green point light source and the blue point light source may be successively turned on for irradiating the photosensitive recording medium individually with red linear light, green linear light and blue linear light.
The light source device (one point light source) may emit light including red light, green light and blue light. In that case, it is preferable that the liquid crystal shutter includes a plurality of individual shutter portions. For example, the plurality of shutter portions include a plurality of first shutter portions for selectively passing red light, a plurality of second shutter portions for selectively passing green light, and a plurality of third shutter portions for selectively passing blue light. Preferably, the one or plurality of point light sources may comprise LED bare chips. In that case, the area of the substrate required for mounting the light source is smaller than that required for mounting a resin-packaged light source, so that the thickness of the light source device is prevented from increasing.
Preferably, the light entrance side of the liquid crystal shutter is covered with a light shielding layer formed with a through-hole for limiting light entering the liquid crystal shutter.
With such a structure, the light with a large incident angle is unlikely to pass through the through-hole to reach the liquid crystal shutter, whereas the light with a small incident angle is likely to pass through the through-hole to reach the liquid crystal shutter. Therefore, the light reaching the liquid crystal shutter has a high directivity, which makes it possible to properly irradiate the photosensitive recording medium with light.
A light diffusing portion may be provided between the illuminator and the liquid crystal shutter.
In the light diffusing portion, light is diffused while the light incident on the light emitting surface at an angle smaller than the critical angle for total reflection is emitted. Therefore, light emitted from the light diffusing layer has a low emission angle and a high directivity. By diffusing light in the light diffusing portion before entering the liquid crystal shutter, it is possible to eliminate the variation in the amount of light, which may initially exist due to the existence of a portion emitting a smaller amount of light in the light source, for example.
Preferably, the light emitting portion includes a projection for coming into engagement with the photosensitive recording medium and a recess for emitting light in the form of a line. With such a structure, when the print head is moved relative to the photosensitive recording medium in close contact with the photosensitive recording medium, it is possible to remove the deflection of the recording medium for preventing defocusing. Further, the sliding resistance between the photosensitive recording medium and the print head can be decreased. As a result, it is possible to smoothly move the print head relative to the photosensitive recording medium, while preventing both the photosensitive recording medium and the print head from being damaged for maintaining the quality of printing.
Preferably, the print head of the present invention further comprises a frame having a predetermined thickness and elongated in the primary scanning direction for supporting the illuminator and the liquid crystal shutter. Preferably, the illuminator and the liquid crystal shutter are elongate in the primary scanning direction, and the illuminator is stacked on the liquid crystal shutter to provide a stack unit, and the stack unit is supported in close contact with the frame at a position deviated thicknesswise from a center of the frame.
Since the illuminator and the liquid crystal shutter are generally elongate in the primary scanning direction, each of these members by itself has a low flexural rigidity against a load in the thickness direction. However, when the illuminator and the liquid crystal shutter are combined to provide a stack unit and the stack unit is held by the frame, the flexural rigidity of the print head is enhanced. Therefore, the print head can be prevented from warping or flexing. Further, when the stack unit is supported on the frame at a position deviated from the center of the frame in the thickness direction, the stack unit is reinforced by the frame, which further enhances the flexural rigidity of the entire print head.
When the flexural rigidity is increased by the use of the frame, the print head can be made thin while avoiding the warping or flexing, which contributes to the size reduction of an image forming apparatus or a digital camera incorporating the print head. Further, when the print head is prevented from warping or flexing, proper light irradiation of the photosensitive recording medium can be performed. This holds true even when the pixel pitch is reduced for realizing high density recording. According to the present invention, therefore, an image with high resolution can be formed.
Preferably, the print head according to the present invention further comprises a lens array including a plurality of lenses aligned in a direction perpendicular to their lens axes. Preferably, in this case, the lens array is held between the stack unit and the frame with the lenses aligned in the primary scanning direction while the lens axes extending in the secondary scanning direction. With this structure, the direction of light traveling through each lens of the lens array extends perpendicularly to the thickness direction of the frame (i.e. extends in the secondary scanning direction). Therefore, the use of the lenses does not greatly increase the thickness of the print head. Further, by disposing the lens array between the stack unit and the frame, the rigidity of the entire print head can be increased.
Preferably, in the print head provided with a lens array, light is emitted from the stack unit for traveling thicknesswise of the frame and the light enters the lens array after its traveling direction is changed by 90 degrees or substantially 90 degrees. Light emitted from the lens array changes its traveling direction by 90 degrees or substantially 90 degrees. For example, the traveling direction of the light emitted from the lens array may be changed by a prism provided with a light emitting portion by 90 degrees or substantially 90 degrees.
Preferably, the prism may include a light incident surface for entrance of light traveling from the lens array, and the light incident surface may be formed with a recess extending in the primary scanning direction.
The lens array may be held by the frame with the plural lenses aligned in the primary scanning direction while the lens axes extending thicknesswise of the frame. In this case, the light emitting portion is provided at a bar-like member elongated in the primary scanning direction and held by the frame. The bar-like member may include a projection for coming into engagement with the photosensitive recording medium and a recess for emitting light in a line. Preferably, in this case, the bar-like member may be held by the frame with the projection projecting from the frame.